1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD), an optical element, a method of fabricating the LCD and a method of making the optical element. The present invention also relates to a material of a liquid crystal alignment film that can be used effectively in the LCD or the optical element.
2. Description of the Related Art
LCDs are used extensively today in portable telecommunications units such as cell phones, personal computers, word processors, amusement appliances, flat panel displays for TV sets, for example, and display boards, windows, doors and walls by utilizing the optical shuttering effects of their liquid crystal layer.
An LCD conducts a display operation by utilizing the optical anisotropy of its liquid crystal molecules. Accordingly, in an LCD, the orientation directions of the liquid crystal molecules are controlled. The orientation directions of liquid crystal molecules may be controlled by any of various techniques depending on the display mode (e.g., TN mode) of the LCD. Normally, the orientation directions of liquid crystal molecules are controlled by using at least one alignment film.
An alignment film has often been either a film of a polymer material such as polyimide or polyvinyl alcohol that had been subjected to be rubbing treatment or a silicon dioxide film deposited by an oblique evaporation process. Among other things, a polyimide film that has been subjected to a rubbing treatment is normally used today as an alignment film. This is because the rubbing technique contributes effectively to mass production and because polyimide is more stabilized chemically (i.e., resulting in a relatively small amount of impurities mixing into the liquid crystal layer) than any other candidate material.
Recently, to meet high demands for development of high-response-speed liquid crystal displays, various orientation modes have been proposed as alternatives to the conventional twisted nematic (TN) mode. Among other things, an optically compensated birefringence (OCB) mode has been researched and developed particularly vigorously as an orientation mode promising high response speed. FIGS. 7A and 7B schematically show exemplary structures of an LCD 400 operating in the OCB mode (which will be herein referred to as an “OCB-mode LCD”). Although not shown in FIG. 7A or 7B, an OCB-mode LCD normally further includes an element for compensating for a phase difference (see Japanese Laid-Open Publication No. 11-271759, for example).
In the OCB-mode LCD 400, the orientation states of its liquid crystal molecules 12a (of a liquid crystal material having positive dielectric anisotropy) are controlled by alignment films 41a and 41b, which are provided on the inside surfaces of two substrates 10a and 10b so as to be in contact with a liquid crystal layer 12, while no voltage is being applied to the liquid crystal layer 12. In that situation, the liquid crystal molecules 12a exhibit a splay orientation state as shown in FIG. 7A. On the other hand, when a voltage that is equal to or higher than a certain voltage Vcr is applied from electrodes (not shown) on the inside surfaces of the substrates 10a and 10b to the liquid crystal layer 12, the liquid crystal molecules 12a exhibit a bend orientation state as shown in FIG. 7B. The LCD 400 can conduct a display operation at a response speed of several milliseconds while the liquid crystal molecules 12a are exhibiting the bend orientation state.
In the OCB-mode LCD 400, however, it is difficult to make all liquid crystal molecules 12a change from the splay orientation state into the bend orientation state uniformly over the entire display area.
It is known that the probability of occurrence of this transition is closely correlated to the pretilt angle (see N. Nagae et al., “A novel method for high speed transition from splay to bend alignment in the OCB-mode LCD with fast response”, IDRC 2000, p. 26, for example). Specifically, if liquid crystal molecules have a pretilt angle of less than 45 degrees, the splay orientation state has a lower Gibbs free energy and is more stabilized than the bend orientation state. On the other hand, when the pretilt angle exceeds 45 degrees, the bend orientation state is more stabilized than the splay orientation state. In the splay orientation state, the smaller the pretilt angle of liquid crystal molecules, the less likely the liquid crystal molecules change into the bend orientation state. Accordingly, in such a situation, a high voltage needs to be applied to the liquid crystal layer to make the liquid crystal molecules change into the bend orientation state.
To make the liquid crystal molecules change into the bend orientation state at a lower voltage more easily, a method of providing a high pretilt angle region for a non-display area on a substrate was proposed. According to this method, the liquid crystal molecules are given a greater pretilt angle in the non-display area than in the display area on the same substrate. In that case, the liquid crystal molecules in the non-display area (i.e., the high pretilt angle region) change into the bend orientation state responsive to a lower voltage applied than the liquid crystal molecules in the display area (i.e., a low pretilt angle region). And those liquid crystal molecules that have changed into the bend orientation state in the high pretilt angle region can be used as cores for making the liquid crystal molecules in the low pretilt angle region change into the bend orientation state more easily.
For example, according to the method disclosed in Japanese Laid-Open Publication No. 2000-75299, the high pretilt angle region is formed by using a vertical alignment film and the low pretilt angle region is formed by using a horizontal alignment film. That is to say, by selectively coating a portion of the surface of a substrate with a different alignment film material from that applied to the remaining portion of the substrate surface, the high and low pretilt angle regions can be formed. Also, Japanese Patent Application No. 2000-107910 discloses a method of forming a horizontal alignment region in a portion of a vertical alignment film by exposing that portion of the vertical alignment film to an ultraviolet ray having a wavelength of 245 nm (which will be herein referred to as a “deep UV ray”).
Not only the OCB-mode LCDs but also LCDs operating in a hybrid aligned nematic (HAN) mode, which is one of the electrically controlled birefringence (ECB) modes, have been researched vigorously. An LCD operating in the HAN mode will be herein referred to as an “HAN-mode LCD”. The HAN-mode LCD utilizes a hybrid orientation state of liquid crystal molecules. Accordingly, compared to a TN-mode LCD utilizing the twisted orientation state of liquid crystal molecules, the HAN-mode LCD excels in high speed response. In addition, unlike the OCB-mode LCD, the HAN-mode LCD need not make the liquid crystal molecules change from the splay orientation state into the bend orientation state. Accordingly, the HAN mode is expected to be a display mode that contributes to driving the LCD at a low applied voltage.
FIG. 8 schematically shows a structure for an HAN-mode LCD 500. In the HAN-mode LCD 500, a horizontal alignment film 51a is provided on the surface of one substrate 10a so as to face a liquid crystal layer 12, while a vertical alignment film 51b is provided on the surface of the other substrate 10b so as to also face the liquid crystal layer 12. As schematically illustrated in FIG. 8, the vertical alignment film 51b includes side chains (i.e., vertical alignment components) 51b′ that extend approximately along a normal to the surface of the substrate 10b. 
If the horizontal and vertical alignment films 51a and 51b are made of different materials, then the films 51a and 51b will exhibit mutually different electrical characteristics (e.g., polarizations). Accordingly, while the LCD is driven by applying a voltage to the liquid crystal layer 12, charges are stored in the alignment film(s) 51a and 51b. In that case, a so-called “DC offset voltage” is generated responsive to the voltage applied to the liquid crystal layer 12. As a result, the image to be displayed is sometimes not refreshed, thus causing a so-called “image persistence” problem.
To overcome a problem like this, Japanese Laid-Open Publication No. 11-311788 discloses a method of forming a horizontal alignment film by subjecting a vertical alignment film to a horizontal alignment process. This publication discloses an exemplary horizontal alignment process in which the vertical alignment film is exposed to a polarizing ultraviolet ray that falls within the wavelength range of 230 nm to 400 nm at a radiation energy of 10 J/cm2 to 20 J/cm2. The ultraviolet ray preferably falls within the wavelength range of 240 nm to 330 nm according to the publication.
The conventional methods described above, however, have the following drawbacks.
As for the OCB-mode LCD fabricating method disclosed in Japanese Laid-Open Publication No. 2000-75299 in which the high and low pretilt angle regions are made of dissimilar materials on the surface of one substrate so as to face the liquid crystal layer, the process steps of applying and patterning the additional alignment film material should be performed, thus increasing the number of manufacturing process steps required and decreasing the throughput.
Also, the present invention discovered and confirmed via experiments that the method of forming a horizontal alignment region by exposing a vertical alignment film to an ultraviolet ray having a wavelength of 245 nm as disclosed in Japanese Patent Application No. 2000-107910 created instability in the orientation state (e.g., the magnitude of the pretilt angle) of the horizontal alignment region. A similar problem also happened even when a vertical alignment film was exposed to a polarizing ultraviolet ray as disclosed in Japanese Laid-Open Publication No. 11-311788.